The present invention pertains to improvements in the field of dynamic printing. More particularly, the invention relates to an improved multicolor electrocoagulation printing method and apparatus.
In U.S. Pat. No. 5,538,601 of Jul. 23, 1996, Applicant has described a multicolor electrocoagulation printing method and apparatus in which use is made of a single positive electrolytically inert electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. Where the coloring agent used is a pigment, a dispersing agent is added for uniformly dispersing the pigment into the ink. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated colloid adhered to the surface of the positive electrode.
In order to provide a polychromic image, the negative electrodes, the positive electrode coating device, ink injector, rubber squeegee and positive electrode cleaning device are arranged to define a printing unit and several printing units each using a coloring agent of different color are disposed around the positive cylindrical electrode to produce several differently colored images of coagulated colloid which are transferred at respective transfer stations from the positive electrode active surface onto the substrate in superimposed relation to provide the desired polychromic image. The substrate which is in the form of a continuous web is partially wrapped around the positive electrode and passed through the respective transfer stations for being imprinted with the differently colored images in superimposed relation.
Since the paper web is brought into contact with the dots of colored, coagulated colloid produced by each printing unit, by the positive cylindrical electrode upon rotation thereof and pressed against the positive electrode active surface by pressure rollers for being imprinted with differently colored images of coagulated colloid, the web is often displaced between the positive electrode and the pressure rollers in a direction parallel to the longitudinal axis of the positive electrode. Accordingly, it is difficult to provide a polychromic image in which the differently colored images are perfectly superimposed.